This invention relates to a sound reproduction arrangement, and to audio signal delay apparatus particularly, but not exclusively, suitable for use in such an arrangement.
When a sound from a primary source is repeated at intervals from other sources which are displaced in azimuth from the primary source, a listener can have the sensation of being in an enclosed space, the reason being that an enclosed space produces reflections similarly delayed and displaced with respect to the direct sound. The character of the sensation, and the apparent size and shape of the space, depend upon the delay intervals for the respective repetitions, the relative intensities of the repetitions, and the directions from which the repetitions approach the listener. It is this effect which produces the sensation of spaciousness noticed by listeners in a concert hall, but which is lacking when, for example, recorded music is reproduced in a domestic living room. The effect can be described loosely as "reverberation," although as will appear below the term "reverberation" is used herein, and in the art generally, in a somewhat more restricted and speciallized sense.
The pattern of reflections in a concert hall can be considered to have two aspects: the early reflection pattern formed by low-order reflections arriving at a listener within a time period, for an average size concert hall, of the order of 100 msec. following arrival of the direct sound; and the "reverberation" pattern comprising large numbers of temporally closely spaced reflections arriving during a period following the early reflections. In most concert halls, the density and randomness or incoherence of reflections in the reverberation pattern is sufficient to resemble band-limited noise with spectral characteristics similar to those of the original sound, but without distinct time- or direction-ordered components. This noise-like reverberation decays generally exponentially at a rate depending upon the physical properties of the hall, those properties often being regarded as the primary acoustic properties of the hall.
In the art, many attempts have been made to artificially reproduce with, for example, recorded music, the sensation of listening to the music in a large enclosure such as a concert hall. Arrangements using various forms of delay device, particularly recirculating delay devices, have been used extensively to mix the primary sound with delayed repetitions of that sound, but in general such arrangements have been successful only in simulating the reverberant aspect of a reflection pattern. Whilst with the use of such arrangements the tonal quality of the reproduced sound seems to correspond reasonably closely with that which would be experienced in a concert hall, the sensation of spaciousness associated with a concert hall is seldom apparent.
It is believed that the production and character of the sensation of spaciousness is critically dependent upon the delay, direction and relative intensity of each of the early reflections heard (although usually not consciously perceived) by the listener. It follows, therefore, that to produce a sensation of spaciousness multiple repetitions should be provided each with an appropriately and individually selected time delay, direction and intensity.
It will be appreciated that audio signal delay apparatus suitable for generating the delayed signals in an arrangement for reproducing the early reflection pattern as described above should preferably be capable of producing a relatively large number, for example ten or more, delayed signals corresponding to a primary input signal. Moreover, the delay constant for each of the delayed signals should be selectable independently of that selected for each of the other delayed signals, should preferably be selectable from within a range of from a few tens of milliseconds to several hundred milliseconds, and should preferably be capable of a relatively fine degree of incremental adjustment, for example, in steps of the order of 1 millisecond or less.